1. Field of Invention
The present invention relates to a power supply method of a line interactive uninterruptible power supply (UPS), and more particularly to a line interactive UPS that employs a bi-directional AC/AC power converter in association with the AC delta control theory to perform the voltage boost/buck conversion.
2. Related Art
The early uninterruptible power supply (UPS) devices mainly include two types, the on-line type UPS and the off-line type UPS. In recent years, a new kind of UPS, the line interactive type, is developed and possesses both the features of the on-line and off-line types. The three kinds of UPS devices are briefly discussed hereinafter.
1. On-line UPS: When the line voltage (local voltage) is normal, the line voltage input to the UPS through a filtering circuit and a noise absorbing circuit will charge a battery set through a charging circuit and input into an inverter through a rectifying circuit simultaneously. After the line voltage is rectified and converted by the inverter, the stable power with superior quality is further provided to electrical instruments, such as a computer or its any peripheral products. Otherwise, when the line voltage is abnormal, the power for those electrical instruments is provided by the charged battery set.
2. Off-line UPS: The line voltage is directly provided to a load without needing to be processed by the UPS when the line voltage is normal. Meanwhile, the line voltage is input through a charging circuit to charge a battery set. When the line voltage is unstable or interrupted, a switch provide inside the UPS switches the power supply path from line voltage to the battery set, whereby the load still can obtain power through the charged battery.
3. Line interactive UPS: Basically, a line interactive UPS is similar to the off-line UPS mentioned foregoing. The difference is that the line interactive UPS will convert the line voltage under a boost (step-up) mode or a buck (step-down) mode, when the line voltage is higher or lower than a normal and still in a predetermined tolerable range. Therefore, the converted voltage is able to provide to the load, and the UPS system does not need to operate in a battery power supply mode.
Usually, the line interactive UPS is equipped with an automatic voltage regulation transformer (AVR TX) therein. When the line voltage coupled to the input of the AVR TX is abnormal, the AVR TX automatically and appropriately switches to one of multiple output taps based on the voltage level of the line voltage. After the line voltage is transformed into a normal level via the AVR TX, the transformed line voltage is output through the appropriate output tap to the load. Since the load is still powered by the line voltage not the battery set, the discharge times of the battery set is accordingly reduced to prolong the use life of the battery set. However, the drawbacks of the line interactive UPS are:
(1) The AVR TX occupies a huge space because of its large size.
(2) The output voltage of the AVR TX is determined by multi-stage control means, i.e. the output voltage is output via one of the multiple output taps. Therefore, it is difficult to precisely determine the output voltage level.
(3) The voltage transforming efficiency of the AVR TX is quite low.
(4) If the tolerable adjust ranger for the abnormal line voltage is intended to widen, the AVR TX must designed with more input taps and output taps. Thus, the complexity of the AVR TX design is raised and the production cost for the entire UPS is accordingly increased.
To overcome the shortcomings, a power supply method of a line interactive UPS and the UPS device in accordance with the present invention obviates or mitigates the aforementioned drawbacks.